In order to even receive a Commendation, you have to actually stay in the dungeon until people vote/leave (so wait for loot to distibute at least). You might just personally be leaving too fast...or others might be leaving without voting. Not everyone will vote, and not every party will vote for you. I found that out after luckily a person in my DF party happened to be on my server and was able to PM via search 'Hey I wanted to vote you up but you left too fast :/'